Thread coupling for drill string elements for percussive drilling

ABSTRACT

A thread coupling for percussive drilling has a first drill string element (2) with an internal thread (3) and a second drill string element (6) with a spigot (7) which is provided with an external thread (8) for cooperation with the internal thread (3). A contact surface (10) on the second drill string element (6) is lined with a wear resistant additional material which covers 50-100%, preferably, 50-90%, of the contact surface. The wear resistant material is preferably a stellite which has been applied by means of plasma welding, or is a ring which has been soldered to the contact surface.

The present invention relates to a thread coupling for drill stringelements for percussive drilling.

In earlier efforts to prolong the service life of the drill tubes atpercussive drilling, see U.S. Pat. No. 4,968,068, it is ascertained thatthe spigot on one of the drill string elements is somewhat shorter, 0-1mm, than the distance between the free end and the bottom surface of thesleeve formed part of the other drill string element. The reason forarranging this clearance at the manufacturing is that one wants toassure that contact is not achieved only between the outer end of thespigot and the bottom surface of the sleeve formed part. It is also saidthat the ideal condition is that one has contact both at the free endand the bottom surface of the sleeve formed part. This condition cannotbe achieved at the manufacturing because of manufacturing tolerances. Itis also said that the original bottom clearance between the two drillstring elements will be eliminated because of wear before 20% of theservice life of the thread coupling has been used. The goal according toU.S. Pat. No. 4,968,068 is thus to keep the original bottom clearancesmall so that one quickly obtains the double contact between the twodrill string components. This results in the drawbacks that thehardening at the free end of the sleeve formed part and the cooperatingcontact surface is badly used since one very soon transfers the wear tothe threads of the two drill string elements.

The present invention, which is defined in the appended claims, aims atincreasing the service life of the drill string elements through lininga contact surface on one of the drill string elements or the free end ofthe other drill string element with a wear resistant additionalmaterial, e.g. a stellite, applied by means of welding or soldering. Itis also an aim of the present invention to obtain the increase of theservice life at a substantiallly lower cost.

Two embodiments of the invention are described below with reference tothe accompanying drawings in which FIG. 1 shows a first embodiment of athread coupling according to the invention, partly in section FIG. 2shows a part of a thread coupling according to a second embodiment ofthe invention.

The embodiment of the invention shown in FIG. 1 comprises a first drillstring element 2 and a second drill string element 6. The first drillstring element 2 is provided with an internal thread 3 and a free end 4.The second drill sting element 6 is provided with a spigot 7 having anexternal thread 8. At the inner end of the spigot the second drillstring element is provided with a contact surface 10. The contactsurface 10 and the free end 4 are situated in a plane beingperpendicular to the longitudinal axis 11 of the thread coupling and areintended for abutting each other. The annular contact surface 10 isprovided with a lining of a wear resistant additional material. It hasturned out to be suitable to choose a stellite which makes it possibleto keep the porosity low. An example of such a material is stellite 6.The material should have about the same hardness as the underlyingmaterial, i.e. H_(RC) =35-45, and cover 50-100%, preferably 50-90%, ofthe contact surface. It har turned out to be particularly advantageousif the wear resistant additional material does not extend entirely outto to the external diameter of the drill string element since thisdecreases the risk that the wear resistant additional material falls offduring operation. The depth of the lining should be 0.5-5 mm. It hasturned out to be particularly advantageous to apply the wear resistantmaterial by means of plasma welding.

FIG. 2 shows a part of a modified embodiment of the thread coupling. Thewear resistant material is here made as a ring 13 which has beensoldered to the contact surface 10. As material a hard metal, e.g. acarbide containing 5-20% Co, is chosen. The embodiment is in otherrespects made as in the example in FIG. 1.

We claim:
 1. Thread coupling for percussive drilling comprising a firstdrill string element (2) provided with an internal thread (3) at a freeend (4) and a second drill string element (6) provided with a spigot (7)which is provided with an external thread (8) for cooperation with theinternal thread (3) on the first drill string element (2), whereby saidsecond drill string element (6) is provided with a contact surface (10)for cooperation with said free end (4) on said first drill stringelement (2), characterized in that said contact surface (10) or saidfree end (4) is provided with a lining (12) of a wear resistantadditional material which has been applied by means of welding orsoldering, that said wear resistant additional material covers 50-100%of said contact surface (10) and has a depth of 0.5-5 mm.
 2. Threadcoupling according to claim 1, characterized in that said wear resistantmaterial is a stellite.
 3. Thread coupling according to claim 2,characterized in that said wear resistant material covers 50-90% of saidcontact surface (10) and has a maximum diameter which is smaller thanthe outer diameter of said contact surface.
 4. Thread coupling accordingto claim 2, characterized in that said wear resistant material has beenapplied by means of plasma welding.
 5. Thread coupling according toclaim 4, characterized in that said wear resistant material covers50-90% of said contact surface (10) and has a maximum diameter which issmaller than the outer diameter of said contact surface.
 6. Threadcoupling according to claim 2, characterized in that said wear resistantmaterial is a ring (13) which has been soldered to said contact surface(10).
 7. Thread coupling according to claim 6, characterized in thatsaid wear resistant material covers 50-90% of said contact surface (10)and has a maximum diameter which is smaller than the outer diameter ofsaid contact surface.
 8. Thread coupling according to claim 6,characterized in that said ring is made of a hard metal comprising 5-20%Co.
 9. Thread coupling according to claim 8, characterized in that saidwear resistant material covers 50-90% of said contact surface (10) andhas a maximum diameter which is smaller than the outer diameter of saidcontact surface.
 10. Thread coupling according to claim 1, characterizedin that said wear resistant material has been applied by means of plasmawelding.
 11. Thread coupling according to claim 10, characterized inthat said wear resistant material covers 50-90% of said contact surface(10) and has a maximum diameter which is smaller than the outer diameterof said contact surface.
 12. Thread coupling according to claim 1,characterized in that said wear resistant material is a ring (13) whichhas been soldered to said contact surface (10).
 13. Thread couplingaccording to claim 12, characterized in that said wear resistantmaterial covers 50-90% of said contact surface (10) and has a maximumdiameter which is smaller than the outer diameter of said contactsurface.
 14. Thread coupling according to claim 12, characterized inthat said ring is made of a hard metal comprising 5-20% Co.
 15. Threadcoupling according to claim 14, characterized in that said wearresistant material covers 50-90% of said contact surface (10) and has amaximum diameter which is smaller than the outer diameter of saidcontact surface.
 16. Thread coupling according to claim 1, characterizedin that said wear resistant material covers 50-90% of said contactsurface (10) and has a maximum diameter which is smaller than the outerdiameter of said contact surface.